1. Field of the Invention
This invention relates to a process for the enhanced recovery of oil and more specifically, to a chemically enhanced process for recovering oil in which a gas is injected into an oil bearing formation prior to the injection of the displacement fluid.
2. Description of the Prior Art
The use of gas injection in conjunction with conventional secondary recovery methods such as waterflooding is known. For example, U.S. Pat. No. 3,525,395 describes a process in which the field is subjected to cycles of gas pressurization, production, waterflooding and production until oil recovery becomes uneconomical. The gas injection is intended to restore reservoir pressure and drive oil to the production system. U.S. Pat. No. 3,599,717 describes the formation of a free-gas phase followed by a gas/waterflood which process may take place in cycles.
Carbon dioxide is frequently the gas injected since it is miscible with petroleum, particularly relatively high viscosity petroleum. U.S. Pat. No. 3,586,107 discloses that when a reservoir is swept with carbon dioxide followed by a water drive, a portion of the carbon dioxide is trapped in pores reducing the amount of carbon dioxide available for oil displacement. This undesirable trapping of the miscible slug can be avoided by interposing a slug of inert gas between the carbon dioxide and waterflood.
U.S. Pat. No. 3,599,716 teaches an air slug followed by a slug of water containing a surfactant. The air results in oxidation of crude oil to form in-situ surface active components which are adsorbed on reservoir rock in place of the surfactants in the aqueous flood. U.S. Pat. No. 3,811,501 relates to the injection of carbon dioxide and an inert gas in order to form a miscible transition zone followed by a driving agent which may be water containing a surfactant. U.S. Pat. No. 3,893,511 employs alternating slugs of a gas and a foaming agent.
Finally, an improved tertiary oil recovery process is described in U.S. Pat. No. 3,882,940 wherein a gaseous slug is injected after subjecting the formation to a chemical tertiary recovery step such as a surfactant or micellar flood.
Because of the expense of surfactants and other additives such as thickening agents, the additional oil recovered as a result of chemically enhanced recovery techniques is usually insufficient to offset the added production costs. It would, therefore, be highly desirable to improve the efficiency of surfactant or micellar floods while at the same time being able to employ common surfactants even in hostile environments such as high brine concentration and eliminate the need for thickeners in pusher slugs.